Why not make a Kickstarter and make the community decide if it is worth it to develop Affinity Photo for Linux.
I for one would not hesitate for one second to throw money on it. It's about damn time someone make a proper Photoshop alternative for Linux and a proper Linux Photo editor in the first place!
Make an estimate of the cost to port it to Linux, then setup the kickstarter for that amount - I would be very surprised if you did not reach that sum if you make some marketing to your current users.
Above all, it would be VERY CHEAP MARKETING TOO.